1. Field of the Invention
The present invention relates generally to power supply control and more particularly to an apparatus and method for control of a cathode ray tube (CRT) anode power supply voltage during vertical retrace.
2. Description of the Prior Art
In a raster scanned CRT the anode power supply load consists of the CRT beam current. Beam current, proportional to the video signal applied to the CRT cathode, is drawn from the CRT anode power supply as the trace scans the phosphor coated face of the CRT. During vertical retrace, the video signal is blanked resulting in a zero value for beam current and presenting a no-load condition for the anode power supply.
The on-off operation of the CRT beam current results in an upward step fluctuation of the CRT anode voltage during vertical retrace due to the inherent output impedance of a CRT anode power supply. This results in a higher anode voltage at the beginning of a scan than near the end. The anode voltage step fluctuation due to vertical retrace blanking causes a loss in resolution and distortion of the information displayed on the CRT.
Prior art solutions for minimizing step fluctuations in anode voltage sought to minimize closed loop output impedance. Prior art solutions include feedback control in the design of regulated power supplies and increasing the capacity of the power supply output filter capacitors. The prior art solutions have specific limitations. All currently known high voltage power supplies have significant time delays in their input/output transfer functions which limit the speed at which the feedback regulated power supply can respond to near instantaneous changes in output loading. Increasing the capacity of the filter capacitor increases its volume and cost as well as requiring further arc suppression in the event of an arcing condition.
The present invention improves on the prior art by incorporating, into the high voltage CRT anode power supply, a control signal, derived from the vertical synchronization signal, which is used to control the anode voltage during the vertical retrace period. During the vertical retrace period, the anode power supply is turned off resulting in the output capacitor not being charged. At the same time, the beam current load is effectively removed by blanking the video signal. Thus, the voltage across the anode power supply output capacitor remains substantially constant until both anode voltage and beam current are effectively turned on again at the beginning of the next scan cycle.